Strategies used by bonnet macaques (Macaca radiata) to reduce predation risk while sleeping

Sleep results in a decrease in alertness, which increases an animal’s vulnerability to predation. Therefore, choice of sleeping sites would be predicted to incorporate predator-avoidance strategies. The current study, conducted in two national parks in southern India, examined the behaviors adopted by bonnet macaques (Macaca radiata) to reduce the risk of being preyed upon while sleeping. Bonnet macaques from an urban setting with a low predatory risk were included for comparison. The physical characteristics of the sleeping sites in the forest corresponded with features that were most difficult for predators to access; bonnet macaques selected emergent trees with high boles near human settlements. These trees typically overhung water. Within the canopy, individuals slept in huddled subgroups near the terminal ends of branches, preferentially selecting branches over water. Subgroups were generally composed of members of the same age and sex, which likely promoted social bonding. Adult males and females with infants selected branches higher than members of other age and sex categories. The lateral distances of individuals along branches from the main trunk were similar across demographic categories. The size of a subgroup appeared to be limited by the weight a branch could support; lateral distances were maintained by regulation of mean subgroup weight, with heavier individuals forming smaller subgroups. The urban troop slept on the top of a building. Subgroup compositions at the urban site were similar to those at the forest sites. However, subgroup size, not restricted by branch fragility, resulted in larger subgroups than those found in the forest. Our results indicate that bonnet macaques adopted a suite of behaviors that reduced their risk of being preyed upon at night by selecting sleeping sites that minimized predator encounters and by selecting the safest locations within the canopy.